This invention relates to an improved method of preparing ceramic materials to provide a ceramic product having a single phase and a fine particle size. More specifically this invention relates to a method of preparing single phase, fine particle size ceramic materials from mixtures of precursor powders, when at least one of the components of the mixture is a carbonate. Still more specifically, this invention is an improved method of preparing single phase, fine grain copper oxide superconductors from precursor powder mixtures where at least one component of the mixture is an alkali earth carbonate.
High-temperature ceramic superconductors are normally prepared by solid-state reaction of oxides, carbonates, or nitrates. For YBa.sub.2 Cu.sub.3 O.sub.x ("123"), Y.sub.2 O.sub.3, CuO, and BaCO.sub.3 are mixed in appropriate proportions and wet milled for approximately 12 hours to prepare a slurry. The wet milled slurry is dried in air and the dried precursor powder calcined at 890-980.degree. C. for 20 to 100 hours. Intermittent grinding is necessary to obtain relatively phase-pure and homogeneous "123" powders. The high temperatures used in the conventional method can induce formation of liquids and non-superconducting phases such as Y.sub.2 BaCuO.sub.5 and BaCuO.sub.2. Furthermore, the CO.sub.2 released during decomposition of BaCO.sub.3, can react with the other components to form BaCO.sub.3, Y.sub.2 O.sub.3, CuO and Y.sub.2 Cu.sub.2 O.sub.5, depending on the temperature. The presence of these non-superconducting phases, especially at grain boundaries, lowers critical current density (J.sub.c). In addition to producing undesirable phases, the conventional processes are time consuming and produce coarse particles. Partial vacuums have been utilized to calcine powders and to sinter polycrystalline bodies, however, in all cases multiphase materials resulted.
A method has been developed for preparing "123" superconductors from precursor powder mixtures which produces single phase, fine grain materials having good bulk superconducting properties. It has been found that by calcining the precursor mixtures in reduced total pressure under flowing oxygen and carefully controlling the partial pressure of CO.sub.2, it is possible to reduce the calcination temperature and eliminate the several grinding and heating steps usually associated with the preparation of the ceramic superconductors. The low pressure enhances the reaction rates thereby substantially reducing the amount of time required for preparation of the superconductor material. The process is suitable for the preparation of copper and other metal oxide superconductor materials and for a wide variety of other ceramic materials which can be prepared by solid state reaction by calcining precursor powder mixtures in which at least one of the components of the mixture is an alkali earth carbonate.